The present invention relates to an optical transmitter/receiver apparatus with integrated hybrid functions of optical reception and transmission for use in optical fiber communication for transmitting an optical signal, which has been output from a semiconductor laser device, through an optical fiber, and also relates to a method for fabricating the same. The present invention further relates to an optical semiconductor module formed by optically coupling a semiconductor laser device to an optical fiber.
In recent years, a fiber-to-the-user system for transmitting data and multi-channel image information from a center station to a home user by using an optical fiber has been proposed and the implementation of such a system is now under consideration. Such a fiber-to-the-user system requires a plurality of optical receiver apparatuses for simultaneously receiving dissimilar optical signals transmitted by wavelength division multiplexing to the terminal device of a home user and an optical transmitter apparatus for transmitting requests, data and the like from the user's terminal device to the center station.
In an optical transmitter apparatus or optical receiver apparatus used for a fiber-to-the-user system, various types of passive alignment mounting techniques are often employed for the purposes of coupling the apparatus to an optical fiber without operating a light-emitting device or a light-receiving device and thereby reducing the costs thereof.
In order to further reduce the size of such an apparatus and further enhance the performance thereof, technology for integrating optical receiver apparatus and optical transmitter apparatus in a compact package is now in great demand.
In response to such demand, an optical transmitter/receiver apparatus, in which an optical receiver apparatus and an optical transmitter apparatus are integrated in a compact package as shown in FIGS. 37(a) and 37(b), has been suggested (see, for example, 1996 Annual Convention of Institute of Electronics, Information and Communication Engineers in Japan, SC-2-5).
Hereinafter, the conventional optical transmitter/receiver apparatus will be described with reference to FIGS. 37(a) and 37(b).
FIG. 37(a) shows a planar structure of the conventional optical transmitter/receiver apparatus, while FIG. 37(b) shows the cross-sectional structure thereof taken along the line A--A of FIG. 37(a). The conventional optical transmitter/receiver apparatus includes a fiber block 10 and a PLC (planar lightwave circuit) substrate 20 that are joined with each other. The fiber block 10 supports one end of a first optical fiber 11 for transmitting/receiving an optical signal therethrough and one end of a second optical fiber 12 for receiving an optical signal therethrough. On the other hand, the PLC substrate 20 supports: a semiconductor laser device 21 for outputting light on a wavelength band of 1.3 .mu.m; a monitoring light-receiving device 22 for monitoring the output of the semiconductor laser device 21; a first light-receiving device 23 for signal reception for receiving light on the wavelength band of 1.3 .mu.m; and a WDM (wavelength division multiplexed) filter 24 for transmitting light on the wavelength band of 1.55 .mu.m and reflecting light on the wavelength band of 1.3 .mu.m. An optical waveguide 25 is formed inside the PLC substrate 20. A second light-receiving device 13 for signal reception for receiving light on the wavelength band of 1.55 .mu.m and outputting image information is connected to the other end of the second optical fiber 12 for reception.
The fiber block 10 and the PLC substrate 20 are optically coupled to each other at an output port 26 and a common port 27. The light to be transmitted on the wavelength band of 1.3 .mu.m, which has been output from the semiconductor laser device 21, is passed through a Y-shaped branch 25a of the optical waveguide 25, the WDM filter 24 and the common port 27 so as to be output through the other end of the first optical fiber 11. Light on the wavelength band of 1.3 .mu.m and light on the wavelength band of 1.55 .mu.m are input to be received through the other end of the first optical fiber 11. The former light, i.e., light on the wavelength band of 1.3 .mu.m, is passed through the common port 27, the WDM filter 24 and the Y-shaped branch 25a of the optical waveguide 25 so as to be received by the first light-receiving device 23. The latter light, i.e., light on the wavelength band of 1.55 .mu.m, is reflected by the WDM filter 24 and passed through the output port 26 so as to be received by the second light-receiving device 13.
In the conventional optical transmitter/receiver apparatus, the entire coupling, except for the coupling between the first and second optical fibers 11 and 12 (which are external transmission lines) and the PLC substrate 20, is realized by passive alignment.
The conventional optical transmitter/receiver apparatus shown in FIGS. 37(a) and 37(b) uses the PLC substrate 20 as an optical platform. However, if a PLC substrate 20 made of silica material is used, shortening of the length of the PLC substrate 20 is restricted by the minimum bend radius of the waveguide 25. Thus, since the PLC substrate 20 becomes rather long in the direction in which light travels, downsizing of such an optical transmitter/receiver apparatus is hard to realize. That is to say, the waveguide 25 of the PLC substrate 20 has a minimum bend radius, over which loss is caused because of difference in refractive indices between a waveguide layer and a cladding layer. If the difference between the refractive indices is increased, then the minimum bend radius can be decreased. However, in actuality, even when the difference between the refractive indices is increased up to 0.75% (which is the maximum value considering the internal loss of the waveguide and the loss resulting from the coupling with the optical fiber), the minimum bend radius cannot be decreased less than about 5 mm. Thus, in the optical transmitter/receiver apparatus shown in FIGS. 37(a) and 37(b), the required minimum length of the PLC substrate 20 in the optical axis direction is as long as about 15 mm. Since the optical transmitter/receiver apparatus further requires the fiber coupling portion, the resulting length of the apparatus in the optical axis direction becomes 20 mm or more.
Also, in the conventional optical transmitter/receiver apparatus, the light to be received on the wavelength band of 1.55 .mu.m, which has been input into the waveguide 25 of the PLC substrate 20, is output through the output port 26 of the PLC substrate 20 into the second optical fiber 12 and then received by the second light-receiving device 13. Accordingly, cost reduction and downsizing of the optical transmitter/receiver apparatus are adversely restricted.
During the assembly process of this apparatus, a cut recess is provided for the PLC substrate 20 by using a dicing saw, the WDM filter 24 is inserted into the recess, and position and angle of the WDM filter 24 are adjusted. However, since it is difficult to mount the WDM filter 24 with high accuracy, the loss of the light, which is incident through the common port 27 and then travels toward the output port 26, disadvantageously increases.
In addition, when the fiber block 10 is joined with the PLC substrate 20, the first optical fiber 11 and the second optical fiber 12 need to be simultaneously connected to the common port 27 and the output port 26 with high efficiency. Thus, since these parts should be aligned through active alignment, the assembly process is adversely complicated.
Furthermore, mounting process steps requiring high accuracy should be performed when the semiconductor laser device 21 is mounted onto the PLC substrate 20, when the first light-receiving device 23 is mounted onto the PLC substrate 20, when the monitoring light-receiving device 22 is located near the semiconductor laser device 21 and when the first and second optical fibers 11 and 12 are mounted into the PLC substrate 20. Since the number of process steps requiring high accuracy is large, cost reduction is difficult to realize.
In an optical semiconductor module used as an optical transmitter apparatus, a concave groove having a V-shaped cross section and extending in the optical axis direction is formed in a base made of silicon and an optical fiber is installed in the concave groove. In such a manner, the optical fiber can now be mounted on the base with high accuracy.
However, as for mounting of a semiconductor laser device, it is difficult to mount the semiconductor laser device onto the base with high accuracy. This is because electrodes are formed on the semiconductor laser device and the base and the size of the semiconductor laser device is small.
Thus, technology for accurately mounting a semiconductor laser device onto a base by passive alignment is now required. A method for fabricating an optical semiconductor module such as that shown in FIG. 38 is suggested by T. Hashimoto et al., MOC '95, D5, 1995.
As shown in FIG. 38, a concave groove 31, extending in the optical axis direction, for positioning a fiber and a cut recess 32 extending vertically to the optical axis are formed in a base 30 made of silicon. A wiring pattern 33 formed of an Au layer and base marks 34, formed of an Au layer, for alignment are also provided for the base 30. On the other hand, on the reverse surface of a semiconductor laser device 40, a surface electrode (not shown) formed of an Au layer and laser marks 41, formed of an Au layer, for alignment, are also formed. In such a case, the wiring pattern 33 and the base marks 34 of the base 30 are formed during the same process step. Similarly, the surface electrode and the laser marks 41 of the semiconductor laser device 40 are also formed during the same process step. Thus, no special processing is required for passive alignment.
The base 30 is aligned with the semiconductor laser device 40 by making a CCD camera 52 receive infrared rays 51, which have been emitted from an infrared light source 50 and then transmitted through the base 30 and the semiconductor laser device 40, and recognize the base marks 34 of the base 30 and the laser marks 41 of the semiconductor laser device 40 as images.
By installing a singlemode optical fiber 60 in the concave groove 31, the position of the fiber 60 on a plane vertical to the optical axis is determined. And when the fiber 60 comes into contact with a stopper wall of the cut recess 32 (i.e., a wall face closer to the semiconductor laser device 40), the position thereof in the optical axis direction is determined.
However, the conventional optical semiconductor module shown in FIG. 38 has the following problems.
First, a mask for forming the base marks 34 should be aligned with the concave groove 31 of the base 30 such that the base marks 34 and the concave groove 31 are located at the same position in the direction vertical to the optical axis on the plane parallel to the surface of the base 30. However, when the mask alignment is performed, the mask always deviates to some degree. As a result, some misalignment is always caused between the base marks 34 and the concave groove 31. Since the semiconductor laser device 40 is positioned by using the base marks 34 that are already out of alignment with the concave groove 31, the semiconductor laser device 40 is very likely to deviate twofold with respect to the concave groove 31.
In addition, when the CCD camera 52 receives the infrared rays 51 emitted from the infrared light source 50 and recognizes the marks as images, the CAD camera 52 simultaneously observes the base marks 34 and the laser marks 41, which are located away from the CAD camera 52 by mutually different distances. Thus, the image of either mark is adversely defocused and recognized as a blurred image.
The finer the base marks 34 and the laser mark 41 are, the more accurate and precise alignment is realized. However, since the base marks 34 and the laser mark 41 are formed through metal vapor deposition technique, the edges of the base marks 34 and the laser marks 41 are variable on the order of microns. Thus, the image recognition cannot be performed with satisfactorily high precision.
Furthermore, the relative distance between the emission end face of the semiconductor laser device 40 and the laser marks 41 is variable on the order of several microns depending upon the cleavage precision. In the same way, the relative distance between the stopper wall face of the cut recess 32 of the base 30 and the base mark 34 is also variable on the order of several microns depending upon the formation precision of the cut recess 32.
Moreover, according to the conventional image recognition method, mechanical adjustment is performed by recognizing the superposed pattern of the base marks 34 and the laser marks 41 as an image by the use of the infrared rays 51 emitted from the infrared light source 50. Thus, the conventional image recognition method cannot suppress the variation in the relative position between the emission end face and the laser marks 41 of the semiconductor laser device 40 and the variation in the position where the cut recess 32 of the base 30 is formed.
Because of the various problems described above, in the conventional optical semiconductor module, a large degree of misalignment is likely to be caused between the optical axis of the semiconductor laser device and t he optical axis of the optical fiber and the distance between the emission end face of the semiconductor laser device and the incidence end face of the optical fiber is also variable to a large extent.